The present invention relates to an electronic ballast circuit for gas discharge lamps and in particular, but not exclusively for public security or amenity applications.
Electronic ballast circuits have been developed to provide a steady voltage to drive gas discharge lamps or the like. However, due to the fact that the lamp is being driven at high frequencies they suffer from a number of problems, unstable arc, poor crest factor, and high frequency acoustic harmonics which may develop in the circuit, all of which considerably reduces the lifetime of the lamp and the quality of the light output. Commercially available ballasts use a fairly simple way to drive a lamp by high frequency load current through an inductor. Many in the industry are aware of the problems but heretofore there does not appear to have been any commercially available product that fully resolves this problem.
An example of such an electronic ballast is that described in PCT Patent Application No. WO 95/22194. This ballast utilises a square wave voltage waveform to drive a lamp. Such a square waveform sometimes called a driver waveform is recognised as being a particularly suitable waveform for gas discharge lamps. However, it is suggested that this ballast would not solve the above problems in particular by providing a stable arc for lamp operation and preventing high frequency acoustic harmonics.
The high frequency pulsing of a lamp is inclined to produce an unstable arc in the circuit. The arc characteristics can vary every time you switch the lamp on and off again and this happens for every AC cross-over, but at high frequency, this can happen as many as 20,000 times a second and this has a detrimental affect on the arc stability. The arc intensity can oscillate and it also has the affect of reducing the lamp lifetime.
High frequency arc resonance is a problem accepted in the industry. It is believed that having these high frequency resonances can also cause the metal welds to vibrate within the lamp, thus physical structure fatigue will occur at the welding joints. The present invention is directed towards providing an electronic ballast circuit which overcomes the problems inherent in the present methods for driving gas discharge lamps. Essentially, what is required is to provide a lamp load current waveform which will produce a stable arc, a low crest factor and minimise the introduction of high frequency acoustic harmonics.
According to the invention there is provided an electronic gas discharge lamp ballast circuit of the type providing an alternating substantially square waveform characterised in that the ballast includes control means to provide a waveform having a substantially instantaneous transition phase between a current value less than peak currents with a subsequent gradual transition phase to peak current, thus providing a waveform having a substantially vertical rising portion, an upwardly inclined slew rate portion a substantially horizontal steady peak portion a declining slew rate portion and a substantially vertical falling portion followed by a mirror image negative current waveform.
The gradual transition phase may form a substantially straight waveform portion or two substantially straight interconnected waveform portions of varying rate of rise towards the peak portion and decline from the peak portion. Alternatively, the gradual transition phase forms an arcuate waveform portion which can be a sinusoidal portion.
Ideally the instantaneous phase occupies between 25% and 90% of the peak current and preferably between 70% and 80% of the peak current.
The gradual transition phase has a reducing current rate change as it approaches peak current which transition phases occupy between 20% and 60%, or ideally between 30% and 50% of the cycle time.
The invention provides a ballast circuit in which the control means comprises:
a current value sensor;
a signal generator; and
a controller connected between the current value sensor and the signal generator.
The current values sensor and controller in one embodiment includes a micro-controller feeding a digital to analog converter which in turn provides a control signal to the signal generator comprising a pulse width modulator.
The invention also provides a method of driving an electronic gas discharge lamp comprising:
providing an alternating current waveform having a peak and an instantaneous transition between a control value below both positive and negative peak;
sensing the control value causing the current to increase to peak current at a controlled slew rate;
sensing the peak current and maintaining the peak current constant for a preset time interval;
at the end of the pre-set time interval causing the current to reduce from peak current to the control value at the controlled slew rate;
sensing the control value; and
causing the instantaneous transition.
Ideally a low frequency current is provided using high frequency circuitry comprising:
synthesising an input power alternating current waveform using switch mode technology; and
commutating the polarity of the resultant waveform.
Preferably the input power is controlled by using a power factor control to vary the duty cycle and to vary the pulse width of the waveform in response to lamp conditions to maintain peak current within pre-set values.